The current Evolved Universal Terrestrial Radio Access (E-UTRA) Long Term Evolution (LTE) Rel. 8 specification supports up to 4-layer spatial multiplexing. As the enhancement for LTE is coming due to the IMT-Advanced call-of-proposal for yet another generation of upgrade in cellular technology. Thus the various aspects of LTE need to be reevaluated and improved. Of a particular interest is to increase the downlink (DL) peak data rate by a factor of 2 and an increase in the DL spectral efficiency to meet the IMT-Advanced requirements. Since LTE Rel. 8 already supports 64 Quadrature Amplitude Modulation (QAM) and higher-order modulation is infeasible in terms of the error vector magnitude (EVM) requirements, the support of higher-order spatial multiplexing (up to 8 layers) is inevitable.
FIG. 1 shows an exemplary wireless telecommunications network 100. The illustrative telecommunications network includes base stations 101, 102 and 103, though in operation, a telecommunications network necessarily includes many more base stations. Each of base stations 101, 102 and 103 are operable over corresponding coverage areas 104, 105 and 106. Each base station's coverage area is further divided into cells. In the illustrated network, each base station's coverage area is divided into three cells. Handset or other user equipment (UE) 109 is shown in Cell A 108. Cell A 108 is within coverage area 104 of base station 101. Base station 101 transmits to and receives transmissions from UE 109. As UE 109 moves out of Cell A 108 and into Cell B 107, UE 109 may be handed over to base station 102. Because UE 109 is synchronized with base station 101, UE 109 can employ non-synchronized random access to initiate handover to base station 102.
Non-synchronized UE 109 also employs non-synchronous random access to request allocation of up-link 111 time or frequency or code resources. If UE 109 has data ready for transmission, which may be traffic data, measurements report, tracking area update, UE 109 can transmit a random access signal on up-link 111. The random access signal notifies base station 101 that UE 109 requires up-link resources to transmit the UE's data. Base station 101 responds by transmitting to UE 109 via down-link 110, a message containing the parameters of the resources allocated for UE 109 up-link transmission along with a possible timing error correction. After receiving the resource allocation and a possible timing advance message transmitted on down-link 110 by base station 101, UE 109 optionally adjusts its transmit timing and transmits the data on up-link 111 employing the allotted resources during the prescribed time interval.
FIG. 2 illustrates the overall process of downlink communication of the prior art. FIG. 2 illustrates only two input codewords according to the prior art. The baseband signal representing a downlink physical channel is produced by the following steps. Each input codeword to be transmitted on a physical channel has its bits scrambled by respective scrambling circuits 201 and 202. Corresponding modulators 202 and 212 modulate the scrambled bits generating complex-valued modulation symbols. A single layer mapper 203 maps the complex-valued modulation symbols onto one or more transmission layers. Complex-valued modulation symbols d(q)(0), . . . ,d(q)(Msymb(q)−1) for code word q are mapped onto the layers x(i)=[x(0)(i) . . . x(v−1)(i)]T, i=0, 1, . . . , Msymblayer−1 where v is the number of layers and Msymblayer is the number of modulation symbols per layer. Single preceding circuit 204 precodes the complex-valued modulation symbols on each layer for transmission on the antenna ports. Precoder circuit 204 input a block of vectors x(i)=[x(0)(i) . . . x(v−1)(i)]T, i=0, 1, . . . , Msymblayer−1 from layer mapper 203 and generates a block of vectors y(i)=[ . . . y(p)(i) . . . ]T, i=0, 1, . . . , Msymbap−1 to be mapped onto resources on each of the antenna ports, where y(p)(i) represents the signal for antenna port p. Corresponding resource channel mappers 205 and 215 map the complex-valued modulation symbols for each antenna port to resource elements. Corresponding OFDM signal generation circuits 206 and 216 generate complex-valued time-domain the Orthogonal Frequency Division Multiplexing (OFDM) signal for each antenna port.
The most crucial matter in extending the maximum number of layers from 4 to 8 is the extension of the codeword-to-layer mapping or simply termed layer mapping. The extension needs to be backward compatible with LTE Rel. 8 and introduce minimum impact on the current LTE specification especially in terms of control signaling. FIG. 3 illustrates the current layer mapping for two transmitting antennas (2-TX) and for four transmitting antennas (4-TX). For case 311 with one layer and two transmitting antennas, a single codeword CW1 is supplied to preceding for transmission via two antennas. For case 312 with one layer and four transmitting antennas, a single codeword CW1 is supplied to preceding for transmission via four antennas. For case 321 with two layers and two transmitting antennas, two codewords CW1 and CW2 are supplied to preceding for transmission via two antennas. For case 322 with two layers and four transmitting antennas, two codewords CW1 and CW2 are supplied to preceding for transmission via four antennas. For case 323 with two layers and four transmitting antennas, one codeword CW1 is supplied to a serial to parallel (S/P) converter which splits it into two signals and further supplies preceding for transmission via four antennas. As noted in FIG. 3 this case occurs only for retransmission using one codeword when the initial transmission used more that one codeword. For case 332 with three layers and four transmitting antennas, one codeword CW1 is supplied to preceding directly. The second codeword CW2 is supplied to preceding via an S/P converter which splits it into two signals for supply to preceding. In case 332 preceding drives four antennas. For case 342 with four layers and four transmitting antennas, codewords CW1 and CW2 supplies respective S/P converters which each split into two signals and further supply preceding for transmission via four antennas.